Modoc County is located in the far northeast corner of the US State of California, bounded by the state of Oregon to the north and the state of Nevada to the east. As of 2000, its population was 9,449. The current county seat is Alturas, the county's only incorporated city. The county's official slogan is "Where the West still lives."

A large portion of Modoc County is federal reservations. A patchwork of overlapping government agencies form a significant part of the economy and provide services to this rural area. The federal presence includes the following agencies and departments: US Forest Service, Bureau of Land Management, National Park Service, Bureau of Indian Affairs and the US Fish and Wildlife Service.

Native Americans inhabited the county prior to its creation. The Modoc people, who lived at the Klamath River headwaters, were a prominent tribe that broke away from the Klamath tribes to the north in Oregon, and the name of the county was derived from this tribe. The county was home to two other major tribal grouns: the Achumawi (or Pit, for which the Pit River is named), and the Paiute. The first European explorers to visit Modoc County were American explorer John C. Fremont and his traveling party (including Kit Carson) in 1846, who had departed from Sutter's Fort near the confluence of the American and Sacromento Rivers where the city of Sacramento lies today.

The county's Native Americans resisted heavily the colonization of the region by the Europeans. The Modoc War (or Lava Beds War) of 1872–73 brought nationwide attention to Modoc during its protracted battles when a large number of US Army soldiers were unable to overtake a significantly smaller band of Modoc warriors who hid themselves in the lava tubes that are now the Lava Beds National Monument. The War began after the American government made a pretense of purchasing the territory belonging to the Modoc people from the Klamath people, and forced the Modoc people to move to the Klamath Reservation in Oregon. Some Modoc people left the reservation, because the Klamath people made it clear that the Modoc were not welcome there. A companion of Captain Jack shot General Edward Canby at a peacemaking session, leading to the siege at Captain Jack's Stronghold. Native Americans were unfamiliar with siege warfare, and the Modoc surrendered only after they were weakened by starvation.

Undergoing numerous changes in possession, the region was originally placed within Utah Territory until the creation of a separate Nevada Territory. After Nevada was granted statehood in 1864, the region was placed within jurisdiction of Shasta County, and Siskiyou County was, in turn, generated from Shasta County in 1852. Modoc County itself was formed when Governor Newton Booth signed an Act of the California Legislature on February 17, 1874 after residents of the Surprise Valley region lobbied for the creation of a new county from eastern Siskiyou County land. The county residents considered naming the newly formed county after Edward Canby, who had recently perished in battle against the Modoc; the idea of naming the county "Summit" was also considered, but the populace eventually settled on "Modoc." An 1874 vote on the county seat displayed public support for Lake City as the best selection, although the heads of the newly formed Modoc County government decided on Dorris's Bridge instead, where the majority of the county population resided and its location on a series of crossing interstate paths.

Settlement of the county began in earnest in the 1870s, with the timber, gold, agriculture, and railroad industries bringing most of the settlers into the area. The county was a crossroads for the Lassen Applegate Trail which brought settlers north from Nevada to the Oregon Trail and south to trails leading into California's central valley. Early settlers included the Dorris, Belli, Essex, Scherer, Trumbo, Flournoy, Polander, and Campbell families.

Several thousand acres just south of Newell served as the temporary exile for thousands of Japanese-American citizens during World War II at the Tule Lake War Relocation Center, a Japanese American internment camp. A historical marker still stands along State Route 139 in Newell. Tule Lake was the largest of the "segregation camps." On November 8, 2005 Senator Dianne Feinstein called for the camp to be designated a National Historic Landmark.

The county is very diverse geographically. The northwestern edge of the county is dominated by the Medicine Lake Highlands, the largest shield volcano on the U.S. West Coast. The Lava Beds National Monument lies partly within the northwest corner of the County. Also along the western edge of the county is the massive Glass Mountain lava flow. The southwestern corner of the county is a unique ecosystem of isolated hardwoods (oaks) and volcanic mountains with intermountain river valleys.

The northern half of the county is the Modoc Plateau, a 1 mile high expanse of lava flows, cinder cones, juniper flats, pine forests, and seasonal lakes. Nearly 1 million acres of the Modoc National Forest lie on the plateau between the Medicine Lake Highlands in the west and the Warner Mountains in the east. The plateau supports large herds of mule deer, Rocky Mountain Elk, and pronghorn antelope. There are also several herds of wild horses on the plateau. The Clear Lake National Wildlife Refuge and Long Bell State Game Refuge are located on the plateau as well. The Lost River watershed drains the north part of the plateau, while southern watersheds either collect in basin reservoirs or flow into the large Big Sage Reservoir, which sits in the center of the county.

Below the rim of the Plateau is Big Valley in the extreme southwest corner of the county, and the large Warm Springs Valley that forms the bottom of the Pit River watershed that runs through the county. The north fork and south fork of the Pit River come together just south of Alturas. The River collects hundreds of other small creeks as it flows south towards Lake Shasta.

The eastern edge of the county is dominated by the Warner Mountains. The Pit River originates in this mountain range. Hundreds of alpine lakes dot the range, all of which are fed by snow-melt and natural springs. East of the Warner Range is Surprise Valley and the western edge of the Great Basin.

Hot Springs and lava caves are common to Modoc County. There are some geothermal energy resources available in the county, though their viability is highly variable.

A great diversity of plants are found in Modoc County, since this is situated within the biodiverse California Floristic Province. Numerous native trees are found in the county including Garry Oak and Washoe Pine trees. Jeffrey Pine and Ponderosa Pine are also found in large numbers.